1. Field of the Invention
The invention relates in general to hardface coatings, compositions and methods, and, more particularly, embodiments of the present invention relate to hardface coatings, compositions, and methods that relate to spall resistant, low density hardface coatings.
2. Description of the Prior Art
Hardface coatings, particularly chromium and tungsten based coatings formed by the thermal spraying of composite powders are well known, but they are generally prone to spalling, and they are heavy. Thermally sprayed tungsten carbide-cobalt coatings, for example, are very hard, brittle and dense. The formation of coatings by thermal spraying ceramics such as ceramic nitrides had been proposed, but ceramics generally decompose instead of melting. For example, ceramic nitrides decompose at about 1900 degrees centigrade. Thermal spraying operations are typically carried out at temperatures well in excess of 1900 degrees centigrade, so attempts to form coatings by thermal spraying ceramic nitrides had generally been unsuccessful. The application of ceramic nitrides via physical vapor deposition and chemical vapor deposition operations for forming coatings that control wear and friction had been previously proposed, but such vapor deposition operations tended to be slow and expensive.
Previous attempts to improve wear had typically involved making harder and stiffer coatings at the expense of ductility. In general, as the coatings became harder and stiffer, the occurrence of spalling increased.
Prior thermal spray operations for forming hardface coatings typically had as an objective the melting of at least the sprayed material, and often also the surface of the substrate. Thorough melting of the sprayed powder was generally believed to be beneficial and necessary because it improved the prospects for the formation of a metallurgical bond, as distinct from a mechanical bond, between the coating and the substrate. This thorough melting generally resulted in the composition of the coating being more or less uniform throughout. Typical prior thermal spray operations include, for example, HVOF (high velocity oxy-fuel), laser forming, plasma spray, plasma transferred arc, and the like.
Unfortunately, these thermally sprayed coatings, which because of having high hardness, are brittle and are subject to spelling and catastrophic failure when subjected to impacts, point loading, or other high stress situations such as those that exist in landing gear cylinders used in carrier based aircraft. This spallation is caused by intensifying the stress in the high modulus coating, combined with its low strain tolerance. Furthermore, these coatings are very dense, ranging from about 8 grams per cubic centimeter for chrome carbide nickel chrome, and about 16 grams per cubic centimeter for tungsten carbide cobalt coatings. These higher density coatings add substantial weight, have low throughput through HVOF gun systems, and impose significant penalties in fuel economy and payload for aircraft and other transportation systems. Finally, these extremely hard coatings with limited ductility must be diamond super finished to prevent excessive seal wear and eliminate surface flaws that cause early failure. Due to their brittleness and high modulus, they are extremely sensitive to flaws and defects on the surface, and in the coating, meaning they are very difficult to apply, limiting their utility and the number of qualified applicators.
High stress and wear aerospace applications such as aircraft landing gear require a hardface coating on structural elements. Many such applications had previously involved the use of WC—Co coated high strength steels. It has been proposed to replace such high strength steels with titanium alloys, because of the weight savings that could be realized. The titanium alloys have a modulus of elasticity that is less than the previous high strength steels. The previous WC—Co coatings have been found to spall off of the titanium as it flexes. A hardface coating that has a modulus of elasticity low enough to not spall off of titanium is needed. For purposes of weight reduction structural members with thin cross-sections had been proposed. Such structural members tended to flex and deform. This resulted in spalling of the hardface coatings. Again a ductile hardface coating was needed. The formation of a ductile hardface coating previously appeared to be unachievable. Hardness and ductility were generally believed to be unachievable in the same coating.
The use of thermal spray operations to form heterogeneous coatings in which isolated high ceramic content regions are embedded within a ductile matrix is disclosed in Sherman published U.S. application No. 2007/0141270, published Jun. 21, 2007, which is hereby incorporated herein by reference as though fully set forth hereat.
Those concerned with these problems recognize the need for an improved hardface coating.